1. Field of the Invention
This invention relates broadly to a measuring and testing apparatus for measuring the size of a stent in a body vessel and for determining the effect of the stent on surrounding tissue and organs. More particularly, this invention relates to an apparatus and a method for measuring the length of a tubular braided stent needed for use in a blood vessel, and to stent loading and deployment apparatus and methods. The invention also relates to methods for determining whether a stent, when deployed, will block important connecting vessel, and whether such blockage would be detrimental to the patient.
2. State of the Art
Transluminal prostheses are well known in the medical arts for implantation in blood vessel, biliary ducts, or other similar organs or the living body. These prostheses are commonly known as stents and are used to maintain, open, or dilate tubular structures or to support tubular structures that are being anastomosed. When biocompatible material are used as a covering or lining for the stent, the prosthesis is called a stent-graft or endoluminal graft. If used specifically in blood vessels, the stent-graft is known as an endovascular graft. A stent may be introduced into the body by stretching it longitudinally or compressing it radially, until its diameter is reduced sufficiently so that it can be fed into a catheter. The stent is delivered through the catheter to the site of deployment and then released from the catheter, whereupon it self-expands. The contraction to stretching ratio and radial pressure of stents can usually be determined from basic braid equations. A thorough technical discussion of braid equations and the mechanical properties of stents is found in Jedweb, M. R. and Clerc, C. O., "A Study of the Geometrical and Mechanical Properties of a Self-Expanding Metallic Stent--Theory and Experiment", Journal of Applied Biomaterials; Vol. 4, pp. 77-85 (1993). In light of the above, it becomes evident that a stent must possess certain elastic and compression qualities.
A typical state of the art stent, such as disclosed in U.S. Pat. No. 4,655,771 to Wallsten or in U. K. Patent Number 1,205,743 to Didcott, is shown herein in prior art FIGS. 1, 1a, 2, and 2a. Didcott and Wallsten disclose a tubular body stent 10 composed of wire elements 12, each of which extends in a helical configuration with the centerline 14 of the stent 10 as a common axis. Half of the elements 12 are wound in one direction while the other half are wound in an opposite direction. With this configuration, the diameter of the stent is changeable by axial movement of the ends 9, 11 of the stent. Typically, the crossing elements form a braid-like configuration and are arranged so that the diameter of the stent 10 is normally expanded as shown in FIGS. 1 and 1a. The diameter may be contracted by pulling the ends 9, 11 of the stent 10 away from each other as shown by the arrows 16, 18 in FIG. 2. When the ends of the body are released, the diameter of the stent 10 self-expands and draws the ends 9, 11 of the stent closer to each other.
The fact that stents undergo various dimension changes from their compressed form to their uncompressed form, results in complications in placement. Placement of a stent having any degree of elongation and radial force as a result of compression is very difficult for several reasons. First, the stent, depending on its pitch angle, may have to be pushed out of the catheter over a long distance. This may be extremely difficult in light of the increased friction forces and various bent sections encountered in the catheter as it traverses a tortuous path. Second, the stent may conversely shrink significantly in length as its diameter expands, thereby rendering it difficult to accurately place it in a vessel. Third, plaque, thrombus or other protrusions or inclusions in the blood vessel lumen may alter the diameter of the stent which consequently alters the length of the stent. The importance of extreme accuracy in placement of an endovascular graft (EVG) will be appreciated by those knowledgeable in the art. For example, in aneurysmal vessel disease, such as that encountered in the abdominal aorta where the distance between the renal arteries and the aneurysm is quite short (less than 3 cm), misplacement of an EVG over the renal arteries or only in the aneurysm can prove fatal.
Proper placement of the stent becomes impossible where the stent is too long or too short for the body cavity in which it is being deployed. In order to be effective, the dimensions of a vessel must be known very accurately and the stent must be tailored to match the specifications of the vessel.
Several difficulties arise, however, when trying to determine the proper stent length needed for any particular cavity. One such problem, especially present with the self expanding stent design such as described by Wallsten and Didcott, is that it is often difficult to predict exactly to what length the stent should be cut in order to properly fit within a particular blood vessel. For example, when deploying an EVG in an aortic aneurysm, the distal end of the stent may reside in the aneurysmal area if the stent is cut too short in length, thereby not sealing the aneurysm and causing potential problems, such as rupturing of the aneurysm. On the other hand, if the EVG is cut too long, the distal end of the EVG can extend into one of the iliac arteries which will lead to clotting of the contralateral iliac artery. Also, if deployed in a vessel with multiple branching, and EVG which is too long may inadvertently cover an arterial branch, thereby occluding the branch and starving the organ which it is intended to nourish.
It is known to presently approximate the deployment length of an EVG stent by using various angiographical techniques (x-ray examinations of blood vessels or lymphatics following the injection of a radiopaque substance). In particular, this is done by injecting radiopaque dye into a vessel and photographing the dye with an X-ray machine as it moves through the vessel. A shortcoming of this method, however, is that angiography usually produces only two-dimensional views of the vessels being examined which are limited by the plane in which the x-ray is taken. As a result, angiograms often fail to reveal the presence of tortuous paths of the examined vessel which may be going in and out of the plane of the angiogram. In addition, the EVG may expand in the area of the aneurysm, depending on the fibrin (the insoluble protein end product of blood coagulation, formed from fibrinogen by the action of thrombin in the presence of calcium ions) content in the aneurysm, and contract in the narrow areas of the aneurysm, thus rendering any prediction of the necessary stent size difficult. It is also known to use Computerized Tomography (CT) scans and the like to show arterial diameters from which the desired deployment stent length can be extrapolated. The prediction of stent deployment length based solely upon slices of diameter, as well as the non-predictability of the fibrin content in an aneurysm, however, limit the accuracy of CT scans. Other more novel methods for visualizing vessels include spiral CT scan and intravascular ultrasound (IVUS). Besides sharing some of the same disadvantages of angioscopy and CT scans, the spiral CT scan provides an image of the outside of the blood vessel only, and therefore fails to show the inside of the vessel where plaque and thrombus accumulate and where the stent is to be placed. The IVUS suffers from not visualizing the compressibility of fibrin and not providing a readout of vessel diameter and length. Another disadvantage shared by the aforementioned apparatus, is that they only provide instantaneous views of the vessel, and may therefore not be accurately representative of the vessel diameter during systole or diastole of the vessel.
Another problem encountered with stenting, especially with coated stenting (EVG deployment), is that branch arteries are often occluded. For example, when correcting an aortic aneurysm, an EVG is deployed between the neck of the proximal portion of the aneurysm below the renals to the bifurcation, or in the case of a bifurcated EVG, to the iliac arteries or beyond. As a result, the EVG may occlude arteries such as the lumbar arteries, intercostal arteries and even the mesenteric artery. In general, occlusion of these arteries is not detrimental to the patient as the mesentery and the spinal chord are fed by other collateral arteries. In a small number of patients, however, blockage of these arteries can result in paraplegia.